FIG. 7 shows a related art power supply unit.
In the figure, a numeral 20 represents an AC power supply, 21 a switch to switch connection with AC power supply 20, 22 a step-up reactor, 23 a common bus connecting one end of the AC power supply 20 and one end of a load (not shown). A numeral 24 represents an AC-DC converter, 25, 26 diodes, 27 a capacitor connected between a positive pole P and the common bus 23, 28 a capacitor connected between the common bus 23 and a negative pole N, 29 an inverter as a DC-AC converter. A numeral 30 represents a common bus voltage detector circuit for detecting a voltage of capacitors 27, 28, 31 an input voltage detector circuit for detecting a voltage of the AC power supply 20, 32 a current detector circuit for detecting a current of the AC power supply 20, 33 a controller for controlling the AC-DC converter 24.
Numerals 34, 35, 36, 37 represent diodes, 38 a switching element. The diodes 34, 35, 36, 37 and the switching element 38 compose the AC-DC converter 24. The AC-DC converter 24 and the reactor 22 compose a step-up chopper circuit.
A numeral 39 represents a current command generator and 40 a comparator. The current command generator 39 and the comparator 40 compose the controller 33.
Numerals 41, 42 represents switching elements, 43, 44 diodes connected in anti-parallel with the switching elements 41, 42. The switching elements 41, 42 and the diodes 43, 44 compose an inverter 29.
A numeral 45 represents a reactor and 46 a capacitor.
A sign Va represents an input voltage detection value detected by the input voltage detector circuit 31, Vref a reference voltage command, VP, VN bus voltage detection values output from the bus voltage detector circuit 30, ia an input current detection value detected by the current detector circuit 32, i2* a current command output from the current command generator 39.
FIGS. 8 and 9 explain the operation to recharge capacitors 27, 28 in a conventional power supply unit. In the figures, numerals 20 through 28, 32, and 34 through 38 are the same as those in FIG. 7 an the corresponding description is omitted.
Operation of the conventional power supply unit will be described referring to FIGS. 7 through 9.
In the conventional power supply unit, current command generating means 39 of the controller 33 generates a current command i2* from the reference voltage command Vref and the bus voltage detection value VP (or VN) output from the bus voltage detector circuit 30.
A comparator 40 of the controller 33 compares the current command i2* with the input current detection value ia detected by the current detector circuit 32, and in case the input current detection value ia exceeds the current command i2*, turns off the switching signal for turning on/off the switching element 38. In case input current detection value ia lowers the current command i2*, the comparator 40 turns on the switching signal for turning on/off the switching element 38.
By using the switching signal to turn on/off the switching element 38 of the AC-DC converter circuit 24, the AC power of the AC power supply 20 is converted to a DC power when the switching signal is turned on and energy is stored into the reactor 22 to boost the power via the route shown in FIG. 8 or 9, and the sum of the energy stored in the reactor 22 and the AC voltage of the power supply unit is used to recharge the capacitors 27, 28 when the switching signal is tuned off.
By controlling on/off of the switching elements 41, 42 of the inverter 29, a DC voltage charged on the capacitors 27, 28 is converted to an AC power of a predetermined voltage, and the resulting AC power is output.
Next, the operation of recharging the capacitor 27 in case the AC power supply 20 is positive will be described referring to FIG. 8.
In case the AC power supply 20 is positive, the controller 33 turns on the switching element 38 of the AC-DC converter circuit 24 to store energy into the reactor 22 via the route covering the AC power supply 20, switch 21, reactor 22, diode 34, switching element 38, diode 37, common bus 23, and AC power supply 20 in this order. Then, the controller 33 turns off the switching element 38 to recharge the capacitor 27 using the energy stored into the reactor 22 via the route covering the AC power supply 20, switch 21, reactor 22, diode 25, capacitor 27, common bus 23, and AC power supply 20 in this order.
The operation of recharging the capacitor 28 in case the AC power 20 is negative will be described referring to FIG. 9.
In case the AC power supply 20 is negative, the controller 33 turns on the switching element 38 of the AC-DC converter circuit 24 to store energy into the reactor 22 via the route covering the AC power supply 20, common bus 23, diode 35, switching element 38, diode 36, reactor 22, switch 21, and AC power supply 20 in this order. Then, the controller 33 turns off the switching element 38 to recharge the capacitor 28 using the energy stored into the reactor 22 via the route covering the AC power supply 20, common bus 23, capacitor 28, diode 26, reactor 22, switch 21, and AC power supply 20.
FIGS. 10 and 11 show various waveforms in a conventional power supply unit. FIG. 10 shows a case where a light load is applied while FIG. 11 shows a case where a heavy load is applied. In the figures, A shows the waveform of the amplitude of a current command, B the waveform of a bus voltage, C waveforms of an input voltage and a current command, D waveforms of an output current and an output voltage, and E waveform of an instantaneous output power.
In a conventional power supply unit, as described above, by controlling the time the switching element 38 is turned on/off, the voltages of the capacitors 27, 28 are retained to predetermined values. A switching signal to the switching element 38 is instantaneously controlled so that the bus voltage will follow the reference voltage command. Thus, in case an instantaneous load varies to a great extent although an average load in half cycle is constant, the bus voltage varies in synchronous fashion. Thus, in case a response is made earlier, the input current includes a small ripple. In case the load is light, the variation in the bus voltage is small (FIG. 10B). While the load is increased, the bus voltage includes a large ripple voltage (FIG. 11B).
The negative bus voltage VN is the same as the positive bus voltage VP so that the corresponding description is omitted.
As mentioned earlier, in a conventional power supply unit, in case a response is made earlier, a large ripple voltage appears on the bus voltage when an instantaneous load varies widely as the load is gradually increased. Thus an element with a large withstand voltage must be used considering the peak value of a ripple voltage on the bus voltage.
The invention is accomplished in order to solve the problems. The first object of the invention is to provide a power supply unit which allows a bus voltage to follow a reference voltage command while keeping the variation in the bus voltage within a predetermined range regardless of load size.
The second object of the invention is to provide a power supply unit which assures an improved load response and a stable bus voltage even in the presence of an impact load.